1. Filed Of The Invention
This invention relates to a corrosion protection system to be employed on a pipeline crossing and more particularly to a method and apparatus for sealing a cased carrier pipe and introducing an inert gas into a casing annulus about the carrier pipe.
2. Description Of The Prior Art
It has been for many years the common practice in the pipeline industry to protect a pipeline crossing under an obstacle such as a road or a riverbed by providing a casing about the carrier pipe where it passes under the crossing. In such a cased crossing a variety of techniques are used to avoid the corrosion problems associated with electrical current passing between conductive members in the pipeline system. These would include coatings, pipeline wraps, impressed current cathodic protection systems, sacrificial anode systems, to name a few. If in the crossing system the casing and the carrier pipe come into direct contact, a corrosion cell may be set up and at the same time lower the cathodic potential of the carrier pipe near the short, potentially allowing further damage to the pipe.
A typical prior art pipeline crossing system and solution to the problems associated therewith are shown in U.S. Pat. 2,816,575 which describes an apparatus for effecting a water tight seal for the casing pipe ends to prevent ground water carrying alkalines or other harmful salts from entering the casing and setting up direct corrosion or electrolytic action. This system employs a vent open to the atmosphere. One draw back to such an open vent is that moisture in the air comes into the casing annulus though an open vent and may condense on the pipe. This moisture then may become an electrolyte through which external cathodic current can pass from the soil through the electrolyte on to the pipe. The protective cathodic current is dissipated back on to the pipe through this electrolyte resulting in the lowering of protective potentials on the pipe at either end of the casing.
If a pipe is bare or if there is damage to a protective pipe coating that exposes the pipe surface, these bare or otherwise exposed surfaces may be subject to atmospheric corrosion inside the casing. Attempts to eliminate such corrosion have involved the use of dielectric filler or an inhibitor liquid in the annulus, such as a gel. One problem associated with the repair of a road crossing system as in the present application is that of disrupting the use of the carrier pipeline. If removal and replacement of the carrier pipe or cutting of the carrier pipe is required, a great expense is incurred in treating the problem. Therefore, it is important that a remedial system be capable of being retrofitted to the existing cased pipeline crossing if at all possible.
It is, therefore, an object of the present invention to provide a simple and inexpensive pipeline crossing system which can be retrofitted onto a presently installed pipeline crossing and wherein the casing annulus surrounding a carrier pipe is maintained free of electrical leaks between the casing and carrier and further which eliminates the intrusion of moisture into the casing annulus, and still further which permits the escape to the surface through the vent of any product which may leak into other casing annulus.